The prior art is repleted today with vector or field-oriented control of an induction-motor using information derived from the terminal currents to the motor. See for instance IFAC Symposium on Control in Power Electronics and Electrical Drives, Lausanne, Switzerland 1983, Pergamon Press, Oxford 1984;
"Current Inverter in the Sliding Mode for Induction Motor Control," by F. Bilalovic, A. Salanovic, pp. 139-144;
"Field-Oriented Control by Forced Rotor Currents in a Voltage-Fed Inverter Drive," by J. Holtz and S. Stadtfeld, pp. 103-110; and
"Control of AC-Machines with the Help of Micro-electronics," by W. Leonhard, pp. 35-58.
For the purpose of this disclosure, these articles published by Pergamon Press, Oxford 1984, are hereby incorporated by reference.
There are two basic approaches to controlling an AC motor drive. One consists in using voltage and frequency as parameters to adjust the speed of the motor. The drawback here is that for a stepped change in voltage and frequency, the dynamic response of the system is poor. Another approach is the field-oriented control method, which in the past has been implemented in terms of current control, in a close-loop or an open-loop fashion.
The present invention relates to method and apparatus in a field-oriented vector control system for controlling a UFC by providing an AC voltage controlled directly in response to a vector voltage demand applied in terms of magnitude and instantaneous angular position. In the afore-stated incorporated by reference patent application, torque and flux demand were used to generate a voltage demand defined in terms of rectangular coordinates to control an AC motor. With the present invention, the latter approach is extended to UFC systems by converting the voltage demand into vector polar coordinates and using such polar coordinates to control the UFC.
It is known from U.S. Pat. No. 4,482,855 to control a cycloconverter in relation to a current demand and a frequency demand to generate voltage direct and quadrature component demand leading to phase voltage demand for control of the cycloconverter. A cycloconverter, however, is different from a UFC. The former is essentially a restricted frequency changer, in particular since it is naturally commutated. In contrast, the present invention proposes to apply voltage demand control to a UFC. The UFC up to now, was aimed only at controlling frequency. Thus, in the prior art, frequency and time were the two parameters involved in the control of the UFC. The switching sequence was controlled in relation to a frequency demand, and within each switching stage, the time of conduction was established in accordance with a desired output voltage in magnitude.